This invention generally relates to electric discharge lamps and has particular reference to a fluorescent sunlamp which produces controlled amounts of ultraviolet radiation.
Low-pressure type discharge lamps which contain a phosphor that emits radiations in the erythemal portion of the spectrum (the wavelength region of from about 260-320 nm.) are well known in the art. A fluorescent lamp which generates both germicidal and erythemal rays in a predetermined ratio is disclosed in U.S. Pat. No. 3,715,612 issued Feb. 6, 1973 to A. Someya et al. In U.S. Pat. No. 3,764,840 issued Oct. 9, 1973 to H. Shiraishi there is disclosed a fluorescent lamp that produces both visible light and erythemal rays at a ratio which is roughly the same as that in natural daylight. This is achieved by combining two phosphor materials with a bulb that is composed of a soda-lime glass which does not transmit radiations having a wavelength less than 295 nm.
Phosphor materials which efficiently emit ultraviolet radiations in the erythemal region are also well known in the art. U.S. Pat. No. 2,563,900 to Wollentin et al. and U.S. Pat. No. 2,563,901 to Nagy et al. disclose a thallium-activated calcium zinc orthophosphate phosphor and a thallium-activated calcium magnesium orthophosphate phosphor, respectively, which have enhanced outputs of erythemal radiation compared to the thallium-activated calcium orthophosphate phosphor employed in the prior art.
The use of a small amount of finely-divided calcium pyrophosphate (Ca.sub.2 P.sub.2 O.sub.7) as an additive in a fluorescent coating to improve the adherence of the phosphor particles to the glass bulb of a general lighting type fluorescent lamp and also reflect radiations in the 200 to 700 nm region is disclosed in U.S. Pat. No. 3,310,418 issued Mar. 21, 1967 to A. I. Friedman et al. As indicated in columns 1 and 2 of this patent, optimum results were obtained by using calcium pyrophosphate particles of very small particles size (less than about 1 micron in diameter) and by utilizing from 5 to 10 grams of such material in a coating suspension containing 1,000 grams of a halophosphate type phosphor. The calcium pyrophosphate additive accordingly comprised from about 0.5 to 1% by weight of the phosphor content and, since the lamp is designed for use as a general lighting device rather than a sunlamp, it contains only phosphors that emit radiations in the visible region rather than the ultraviolet region of the spectrum.
Sumlamps are rated in terms of E-Viton units. An E-Viton is a quantitative unit of the amount of erythemal ultraviolet radiation which is transmitted by the lamp and is a measure of the effectiveness of the various wavelengths of ultraviolet rays in producing skin reddening. An E-Viton corresponds to the quantity of radiant energy which produces as much reddening of the skin as 10 microwatts of energy at a wavelength of 296.7 nm. Fluroescent lamps of the 40 watt T12 size typically have an output in the range of from around 175,000 to 225,000 E-Vitons.
The phosphors used in fluorescent sunlamps produce ultraviolet radiations having wavelengths which range from about 260 nm to 380 nm. Shorter wavelength ultraviolet radiations generated by the arc discharge and emitted by the lamp (190 nm to 260 nm) are undesirable since they produce very little, if any, tanning of the skin and are suspect from the standpoint of skin cancer. In contrast, the longer wavelength ultraviolet radiations (260 nm to 320 nm) are very effective in producing skin tanning without burning or causing vivid reddening of the skin if exposure times are carefully controlled. As a consumer protective safeguard, the U.S. Government recently established a Federal Performance Standard for sunlamp products which is specified in 21 C.F.R. 1040.20 and requires that the ratio of short-wave emission and longwave emission in the ultraviolet region for such lamps be kept below a certain value. Specifically, the Federal Standard requires that the ratio of short UV radiation (190-260 nm range) to long UV radiation (260-320 nm range) be less than 0.003.
Lamp tests have demonstrated that the aforementioned performance standard can be met by increasing the density of the phosphor coating of a conventional fluorescent sunlamp through the use of higher powder weights (that is, thicker phosphor coatings). However, it was found that small changes in the coating density drastically affect the output of the sunlamp within the long wavelength region (260-320 nm range) and that the use of larger amounts of phosphor increased the E-Viton output to a value above that desired for safe and effective skin tanning. In addition, the use of heavier phosphor coatings to reduce the amount of short UV radiation that is transmitted by the lamp inherently increases the amount of phosphor required per lamp and adds to the manufacturing cost.